This invention relates to electronic apparatus, and more particularly to apparatus having one or more integrated circuit chips affixed to an electrically insulating substrate.
Integrated circuit chips are now a major factor in the fabrication of electronic devices. In particular, multichip modules are in common use. Such modules are frequently constructed by affixing a number of chips to a dielectric substrate, typically ceramic, and connecting the chips through conductive paths or vias which may pass through holes in the substrate. A typical chip material is gallium arsenide.
Electronic activity in the chip is accompanied by the generation of varying amounts of heat, which is discharged in large part by transmission to and through the substrate. As the packing density of chips increases, the amount of heat which must be discharged naturally also increases. It is usually desirable to employ a substrate material with a high thermal conductivity. Aluminum nitride is an example of a ceramic substrate material having the desired thermal and electrical properties.
A problem arises, however, by reason of the differing degrees of thermal expansion of gallium arsenide and aluminum nitride. Their coefficients of thermal expansion are about 6.5 and 4.5 ppm./.degree. C., respectively. Thus, thermally induced tensile stresses are along the adhesion interface between the chip and the substrate, placing the chip under severe tension.
When the size of the chips, and especially the area of attachment of a chip to the substrate, is relatively small, such thermal stresses are fairly easily discharged and do not present a problem. This is true, for example, in the case of laser diodes whose lateral dimensions are typically on the order of 0.5 mm. However, chips employed for other purposes are increasing in size, typically being as large as 12 mm. in length and/or width with dimensions up to 6 mm. being commonplace. Such chips are also typically very thin, often on the order of 100 microns. When this is the case, the large tensile stresses between the chip and the substrate can result in various detrimental effects such as cracking of the chip or its separation from the substrate.
By reason of its extremely high thermal conductivity and electrical resistivity, diamond is becoming of greater interest as a substrate material for integrated circuits. While single-crystal diamond as exemplified by natural diamond may be used for this purpose, synthetic polycrystalline diamond is more common because of its substantially lower cost. Such diamond may be prepared, for example, by chemical vapor deposition (CVD). However, the thermal expansion difference between gallium arsenide and diamond is even greater than that between gallium arsenide and aluminum nitride, since the coefficient of thermal expansion of CVD diamond is only on the order of 1.5 ppm./.degree. C.
Attempts have been made to overcome this difference in thermal expansion tendency by employing such materials as soft solders and conductive epoxy resins as adhesives to affix the chip to the substrate. Such materials, however, are themselves not fully resistant to the tensile stresses induced. Moreover, their thermal conductivity is much lower than that of diamond, which to some extent defeats the purpose of having a diamond substrate.
A different approach to the thermal expansion problem is described in U.S. Pat. No. 5,324,987. It involves the interposition between the substrate and chip of a plurality of diamond pedestals in combination with areas of other material, typically metal of high thermal conductivity, having a higher coefficient of thermal expansion than said substrate. Despite its success, however, this approach is less than ideal because of the difficulty and expense of machining the diamond pedestals and the necessity to custom-match the substrate to the chips being mounted thereon.
It is highly desirable, therefore, to develop an alternative means for attaching integrated circuit chips, especially gallium arsenide chips, to substrates of high thermal conductivity such as aluminum nitride and diamond. It is further desirable for the attaching material to permit grading of the stresses induced by the large differences in thermal expansion between the chips and the substrate. Said means must be adaptable to soldering and wire bonding operations.